The invention will be described in relation to the video content of the NTSC TV signal. However, NTSC transmission includes gamma (.gamma.) correction for typical CRT beam drive non-linearity. This feature is not pertinent to some color separation methods and it is to be understood that gamma correction will not apply herein except where shown. NTSC is well known and there are numerous methods by which the signal may be separated into three fund mental components of timing, (sync) lumenance (y) and chrominance (C). The NTSC signal may be further converted divided into the display outputs components, which are brightness, or equal brightness, the means by which the eye sees the intensity output of the display, and into its dominant hue (H) and degree of saturation (S), which are the two components by which the eye recognizes color content (its chromaticity) of a scene. The method of generation of the display, however, relies on the provision of three primary colors, red (R), green (G) and blue (B), which have become the universal standard for providing a full range of color content.
Several methods of color display are known in the art. These include the well known shadow mask and CRT's employing beam penetration phosphors. A third system uses a separate CRT for generation of each color and the three components must be mechanically and optically superimposed exactly to register the three outputs to provide a single full color display. These methods each have distinct disadvantages with respect to parameters such as bulk, complexity, and cost and performance characteristics such as luminous output efficiency, resolution, and display accuracy.
Another type of system, most of which were unsuccessful and eventually abandoned, are known as indexing systems. These systems failed, generally, because of technical requirements, the importance of which were not recognized, or because the art was not sufficiently advanced to meet the systems' needs.
A large majority of these indexing systems rely on a technique known as sequential color separation (SCS). SCS depends upon the scan of an electron beam in a cathode ray tube across a sequential pattern of triads of primary color phosphors (e.g., red, green, and blue). The control signals for each of these colors turn on the beam in the proper sequence for each phosphor position, and this must be very precisely controlled to provide faithful color rendition. The function of indexing is to provide this precise beam position control which, in uniform raster scan applications, implies precise time control.
Because SCS implies a single beam which passes sequentially across all colors, even if a particular color is not to be produced, this technique has a fundamental limitation in beam energy efficiency of 17%. This efficiency is comparable to that of the shadow mask techniques and prevented "indexing" from achieving the advantages over other methods which were expected of it. Attempts to use beam velocity modulation to improve efficiency were not compatible with other requirements specifically including the methods for indexing then used. As a result, these SCS systems failed to become competitive.
Other proposed displays departed from SCS but never achieved the reality of production. These had in common the concept of dynamically directing the beam (necessarily at high speeds for TV) only to the colors to be reproduced in each color triad. This concept has the possibility of a beam efficiency reaching 100% and in addition permits use of more than one beam, both thereby providing means to generate a very bright, high resolution output. This concept has basic features distinctly different from SCS, and the term "dynamic color separation" (DCS) is introduced herein to describe this mode of color separation.
The art of indexing-type color receivers has been substantially dormant for many years except for a minor recent revival of interest in small displays using SCS. During this interval, a substantial variety of improvements has been made in fields unrelated to SCS or DCS displays. A substantial number of improvements, related to the B/W functions of the display or to improved signal control means such as solid state and IC circuits, which are desirable to obtain the performance required to make a DCS display successful, have had no suggestion of applicability in this field.
With respect to the color display functions, a principal DCS signal processing function is conversion from input composite video (CV) to luminance Y, and R, G, and B signals, or to equal brightness monochrome M and saturation S signals, and further from S to hue phase (.THETA.) or derived voltage (HV) and saturation voltage (SV) control signals. The first is well known in the art, and the second (M and S) is not so well known but was first derived by B. D. Loughlin, and a detailed description relating them directly to DCS may be found in U.S. Pat. No. 3,312,779 (Washburn).
Prior art employing waveforms providing features essential to DCS in a color display are to be found in U.S. Pat. No. 2,989,582, (Zworykin et al.) which describes excessively complicated ramp waveforms; U.S. Pat. Nos. 2,966,544, 3,004,098 and 3,096,095 (Gargini) which disclose a saturation signal amplitude modulated ramp waveform for DCS; U.S. Pat. No. 3,312,779 (Washburn) which employs both the amplitude modulated ramp and a saturation signal time modulated, dual slope beam arrest ramp for DCS; and U.S. Pat. No. 3,431,456 (Liebscher) which describes a three beam system employing a fixed amplitude beam arrest ramp for DCS.
There are a substantial number of prior art systems which use the chrominance component C of composite video to velocity modulate the beam, thereby to provide a degree of color brightness enhancement. Because C is a function of signal luminance Y, it can not perform color separation which is a function only of display format dimensions and scan timing and which must be independent of display luminance or brightness content. The saturation signal S, which is independent of luminance can provide color separation control independently of luminance Y or monochrome M content of the display. Basic color separation using C, of which U.S. Pat. No. 3,893,165 (Sunstein) is a relatively recent example is, however, still SCS.
There are prior systems which use miscellaneous fixed waveforms for deflection of the electron beam across horizontally oriented stripes, which is also SCS. There is no prior art teaching employing DCS for either of these two examples.
The above DCS techniques all employ frequency or time control to control the position of the beam. Thus, Zworykin uses an index pulse as a direct sync signal (sequentially) at each triad. Gargini develops a heterodyned, or beat frequency, signal (as do many SCS systems) controlled by the index signal system. This method of time/position control, which has been a preferred choice for SCS, conflicts with the modulation of the beam velocity and even more directly with DCS because hue position selection demands selective movement of the beam across the color triad of each pixel. This movement is controlled by the phase shift of chrominance C, or S (independent of luminance), whereas the index signal should generate a reference signal having constant phase. There is no prior art teaching which eliminates this problem.
Sunstein's purpose (as is Gargini's) is to ameliorate this problem, and reference is made to columns 2 and 3 of the above Sunstein patent for a background discussion of the problems of such prior art. Liebscher also describes his three beam system using an index generated frequency control signal. It is readily apparent that when the DCS beam arrest is working satisfactorily, both the amplitude and frequency content of the index signal disappear in this disclosed system. In Washburn, time shifts sequentially between DCS action and position control (APC) action, thereby making the functions perform independently. This method introduces circuit complexity but has the more significant disadvantage that it introduces low frequency dead space into the display. The eye is more sensitive to this low frequency content than to otherwise normal pixel structure. Thus prior art discloses no position control means which are well suited to work with DCS.
In summary, there had been much initial interest in SCS indexing color TV receivers because they appeared to be simpler and to provide higher performance--especially a higher resolution display. SCS, however, proved to have low light output efficiency, and methods to increase light output also increased complexity and cost. The very factors which allow increased resolution also add to the difficulties of achieving high brightness and require high precision in control of the beam to provide output color fidelity. Some basic concepts, especially dynamic color separation (DCS) waveforms which increase brightness efficiency, were introduced. These methods, however, all had deficiencies in generating a satisfactory display which were never recognized or overcome because attempts to develop "indexing systems" were abandoned. This has remained the situation for the past twenty years. During this interval demand for higher performance, resolution, size, brightness, etc. have increased, and economical solutions have not appeared. Recent introduction of various aspects of HDTV is one example of such recent demand.